Therapy-Induced Transdifferentiation Promotes Glioma Growth Independent of EGFR Signaling.

EGFR is frequently amplified, mutated, and overexpressed in malignant gliomas. Yet the EGFR-targeted therapies have thus far produced only marginal clinical responses, and the underlying mechanism remains poorly understood. Using an inducible oncogenic EGFR-driven glioma mouse model system, our current study reveals that a small population of glioma cells can evade therapy-initiated apoptosis and potentiate relapse development by adopting a mesenchymal-like phenotypic state that no longer depends on oncogenic EGFR signaling. Transcriptome analyses of proximal and distal treatment responses identified TGFβ/YAP/Slug signaling cascade activation as a major regulatory mechanism that promotes therapy-induced glioma mesenchymal lineage transdifferentiation. Following anti-EGFR treatment, TGFβ secreted from stressed glioma cells acted to promote YAP nuclear translocation that stimulated upregulation of the pro-mesenchymal transcriptional factor SLUG and subsequent glioma lineage transdifferentiation toward a stable therapy-refractory state. Blockade of this adaptive response through suppression of TGFβ-mediated YAP activation significantly delayed anti-EGFR relapse and prolonged animal survival. Together, our findings shed new insight into EGFR-targeted therapy resistance and suggest that combinatorial therapies of targeting both EGFR and mechanisms underlying glioma lineage transdifferentiation could ultimately lead to deeper and more durable responses. SIGNIFICANCE: This study demonstrates that molecular reprogramming and lineage transdifferentiation underlie anti-EGFR therapy resistance and are clinically relevant to the development of new combinatorial targeting strategies against malignant gliomas with aberrant EGFR signaling.